Convert `StringAgg` to UDAF #28041

Job	Run time
cargo test (macos-aarch64)	9m 26s
cargo test (macos)	9m 31s
build with wasm-pack	2m 21s
Check cargo fmt	36s
Verify MSRV (Min Supported Rust Version)	6m 42s
Verify Vendored Code	51s
cargo test (win64)	32m 23s
cargo check	4m 7s
cargo test (amd64)	9m 18s
cargo examples (amd64)	11m 44s
check Cargo.toml formatting	2m 48s
clippy	5m 20s
cargo test hash collisions (amd64)	9m 1s
check configs.md is up-to-date	2m 50s
cargo doc	3m 39s
cargo test datafusion-cli (amd64)	3m 38s
cargo test doc (amd64)	12m 39s
Run sqllogictest with Postgres runner	3m 17s
cargo test pyarrow (amd64)	2m 39s
verify benchmark results (amd64)	10m 44s
	2h 23m 34s

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